So I got some info from support.

Not to helpful as quite contradictory, and marked as "forwarded to company" so no means of continuing that support conversation.

Support said expressions are not supported everywhere.

The reason divide works everywhere is that it's a fraction (makes sense).

But they pointed me at this article

http://www.ellenfinkelstein.com/acadblog/let-autocad-do-the-math/

which has good info on how to do much of what I wanted, including showing =250+250 in a radius property in the  property inspector. As well as autolisp expressions.

The big problem is none of the things in that article work for me.

Mac OS Sierra

Autocad LT 2017 for Mac

Side note: Support marked as forwarded to company? Sounds like "sent into the either" to me. Anyone ever hear from the company again when support does this?